Metal oxide thin film transistors (MOTFT) are gaining interest as high performance TFT backplanes for large area applications such as active matrix organic light emitting diodes (AMOLED). See for example the copending U.S. patent application entitled “Active Matrix Light Emitting Display” bearing Ser. No. 12/178,209, filed on 23 Jul. 2008, and incorporated herein by reference. Many of these large area applications use either glass or plastic substrates. To produce TFTs on large areas at low cost, it is advantageous to use low cost lithographic tools such as proximity/projection aligners rather than the more expensive stepper tools. Furthermore, due to deformation of substrates in processing (either glass due to high temperature treatment or plastic substrates due to chemical and heat treatment), the alignment issue has to be solved. Typically, misalignment due to deformation increases with the size of the exposure field. One way to compensate for the deformation is to reduce the exposure field by performing multiple exposures on the substrate and then stitching the multiple patterns together. However, this process substantially increases the manufacturing cost due to lower through put and the high cost of stitching.
It would be highly advantageous to have a self-aligned process in which there are no critical alignment steps.
Accordingly, it is an object of the present invention to provide new and improved methods of fabricating self-aligned metal oxide TFTs.
It is another object of the present invention to provide new and improved methods of fabricating metal oxide TFTs including no critical alignment tools or steps and using a minimum of process steps.